The present invention relates to a shearing apparatus for shearing objects, such as a drill string, extending through a wellhead. The drill string may include a kelly joint, drill pipe, tool joints and drill collars of various weights per foot.
During the drilling of a well it may be necessary to shear the drill string at the wellhead to control the well. It is therefore desirable to be able to shear whatever portion of the drill string may be positioned in the wellhead when an emergency condition arises requiring shearing of the string and sealing of the well.
Prior shearing devices, such as shown in U.S. Pat. No. 3,561,526, were in the form of blowout preventers which cut through drill pipe and provided a seal against the well pressure after the well pipe had been sheared. While these shearing devices have functioned satisfactorily in shearing drill string pipe, they have not been designed to shear drill collars or heavy casing for several reasons. Usually they do not have sufficient power for shearing the heavy drill collars. They tend to flatten a casing so that it is wider than the preventer bore and hangs in the guideways. Also, in exerting the force necessary to shear a drill collar, the cutting blades have a tendency to separate vertically which reduces their shearing ability.
Some prior shearing devices provided guides to prevent separation of the blades. U.S. Pat. No. 1,802,564 discloses a wellhead shearing apparatus having slots in the body surrounding the well bore, the blades sliding within such slots in their shearing action. A side support for cutter blades is provided by the device disclosed in U.S. Pat. No. 3,399,728.